1. Field of the Invention
The present invention relates to regenerative adsorption gas dryers and desiccant cartridges for such dryers.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Adsorption dryers for compressed air and gases have been known for many years and are widely used throughout the world. Although other types of dryer are available, such as deliquescent and refrigeration dryers, these cannot give a pressure dew-point as low as that achieved by adsorption dryers.
Normally, adsorption dryers are dual tower dryers. That is to say, they include two towers of desiccant material (commonly known as beds) one of which is “on-stream” drying the gas whilst the other is “off-stream” and being regenerated. In a dual tower dryer, the gas to be dried is passed through the desiccant bed of the on-stream tower continuously, in one direction, during a drying cycle. Then, after a predetermined time interval (this interval being chosen such that the bed will have adsorbed sufficient moisture) the inlet gas is switched to the desiccant bed of the other tower and the first desiccant bed is regenerated by some suitable procedure such as heating, evacuation or passing a purge gas through it, usually in a flow direction opposite to the flow of gas to be dried.
A heatless dryer typically uses a purge flow of dry gas, which is usually a proportion of dried gas from the on-stream tower, the purge gas being passed through the regenerating bed at a lower pressure than the gas in the on-stream tower. The dryer is normally operated on a fixed time cycle for drying and regeneration and both cycles are usually of an equal duration, or they can be operated in a variable cycle. The cycle times for heatless dryers are usually measured in minutes.
To control the flow of gas from one tower to the other, and to control the purge gas, a series of valves is employed. These valves typically include inlet valves which switch the gas from one tower to the other, exhaust valves which control the duration of purge gas flow and repressurisation of the towers, and outlet valves which prevent the outlet stream pressurizingthe off-stream bed. Other valves may also be required.
The applicant's earlier application, EP 1378286, discloses a dryer in which an incoming wet gas stream enters a first manifold at the head of the towers and is then directed to a second manifold at the foot of the towers. A first shuttle valve in the second manifold directs the wet gas stream into the on-stream tower. The gas flows through the tower back to the first manifold where it meets a second shuttle valve which is positioned to allow flow of the dried gas out of the manifold.
As the dry gas flows back into the first manifold from the on-stream tower, a small bleed of the dry gas is channeled off through a purge gas passageway in the manifold to the off-stream tower. The purge gas expands as it passes through orifices in the gas passageway so that, on entry into the off-stream tower, it is substantially at atmospheric pressure. The purge gas is channeled back down to the second manifold through the off-stream tower thus regenerating the desiccant in the tower. The purge gas exits the off-stream tower into the second manifold where it is exhausted from the drier by an exhaust valve for that tower.
The action of the gas passing through the exhaust valves of the towers is responsible for the control of the first shuttle valve in the second manifold. If gas is being exhausted from the exhaust valve of one tower, the first shuttle valve switches to ensure that wet gas yet to be dried is channeled into the other tower. The pressure of the dried gas exiting the other tower then ensures that the second shuttle valve switches to allow flow of the dried gas out of the first manifold.
However, expensive manufacturing processes, such as machining or complex casting, are generally needed to form the manifolds, shuttle valves and gas passageway. Further, locating the shuttle valves and gas passageway in the manifolds can make them difficult to access for maintenance, and can make them expensive to alter if valves and passageways with different flow characteristics are required.